The Role of Magmatic and Hydrothermal Fluids in the Formation of the Sasa Pb-Zn-Ag Skarn Deposit, Republic of Macedonia

geosciences

Article The Role of Magmatic and Hydrothermal Fluids in the Formation of the Sasa Pb-Zn-Ag Skarn Deposit, Republic of Macedonia

Sabina Strmi´cPalinkaš 1,*, Zlatko Peltekovski 2, Goran Tasev 3, Todor Seraﬁmovski 3, Danijela Šmajgl 4, Kristijan Rajiˇc 1,5 , Jorge E. Spangenberg 6, Kai Neufeld 1 and Ladislav Palinkaš 5

1 Department of Geosciences, Faculty of Sciences and Technology, UiT The Arctic University of Norway in Tromsø, N-9037 Tromsø, Norway; [email protected] (K.R.); [email protected] (K.N.) 2 Mine SASA DOO, Rudarska u. 28, MK-2304 Makedonska Kamenica, Republic of Macedonia; [email protected] 3 Institute of Geology, Faculty of Natural and Technical Sciences, University Goce Delcev, MK-2000 Stip, Republic of Macedonia; [email protected] (G.T.); todor.serafi[email protected] (T.S.) 4 Thermo Fisher Scientific, 28199 Bremen, Germany; danijela.smajgl@thermofisher.com 5 Department of Geology, Faculty of Science, University of Zagreb, HR-10000 Zagreb, Croatia; [email protected] 6 Institute of Earth Surface Dynamics, Geopolis, University of Lausanne, CH-1015 Lausanne, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +47-77-625-177

Received: 10 October 2018; Accepted: 15 November 2018; Published: 29 November 2018

Abstract: The Sasa Pb-Zn-Ag deposit belongs to the group of distal base metal skarn deposits. The deposit is located within the Serbo-Macedonian massif, a metamorphosed crystalline terrain of Precambrian to Paleozoic age. The mineralization, hosted by Paleozoic marbles, shows a strong lithological control. It is spatially and temporally associated with the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene time period and resulted in the formation of numerous magmatic–hydrothermal ore deposits. The mineralization at the Sasa Pb-Zn-Ag deposit shows many distinctive features typical for base metal skarn deposits including: (1) a carbonate lithology as the main immediate host of the mineralization; (2) a close spatial relation between the mineralization and magmatic bodies of an intermediate composition; (3) a presence of the prograde anhydrous Ca-Fe-Mg-Mn-silicate and the retrograde hydrous Ca-Fe-Mg-Mn ± Al-silicate mineral assemblages; (4) a deposition of base metal sulfides, predominately galena and sphalerite, during the hydrothermal stage; and (5) a post-ore stage characterized by the deposition of a large quantity of carbonates. The relatively simple, pyroxene-dominated, prograde mineralization at the Sasa Pb-Zn-Ag skarn deposit represents a product of the infiltration-driven metasomatism which resulted from an interaction of magmatic fluids with the host marble. The prograde stage occurred under conditions of a low water + + activity, low oxygen, sulfur and CO2 fugacities and a high K /H molar ratio. The minimum pressure–temperature (P–T) conditions were estimated at 30 MPa and 405 ◦C. Mineralizing fluids were moderately saline and low density Ca-Na-chloride bearing aqueous solutions. The transition from the prograde to the retrograde stage was triggered by cooling of the system below 400 ◦C and the resulting ductile-to-brittle transition. The brittle conditions promoted reactivation of old (pre-Tertiary) faults and allowed progressive infiltration of ground waters and therefore increased the water activity and oxygen fugacity. At the same time, the lithostatic to hydrostatic transition decreased the pressure and enabled a more efficient degassing of magmatic volatiles. The progressive contribution of magmatic CO2 has been recognized from the retrograde mineral paragenesis as well as from the isotopic composition of associated carbonates. The retrograde mineral assemblages,

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represented by amphiboles, epidote, chlorites, magnetite, pyrrhotite, quartz and carbonates, reflect conditions of high water activity, high oxygen and CO2 fugacities, a gradual increase in the sulfur + + fugacity and a low K /H molar ratio. Infiltration fluids carried MgCl2 and had a slightly higher salinity compared to the prograde fluids. The maximum formation conditions for the retrograde stage are set at 375 ◦C and 200 MPa. The deposition of ore minerals, predominantly galena and sphalerite, occurred during the hydrothermal phase under a diminishing influence of magmatic CO2. The mixing of ore-bearing, Mg-Na-chloride or Fe2+-chloride, aqueous solutions with cold and diluted ground waters is the most plausible reason for the destabilization of metal–chloride complexes. However, neutralization of relatively acidic ore-bearing fluids during the interaction with the host lithology could have significantly contributed to the deposition. The post-ore, carbonate-dominated mineralization was deposited from diluted Ca-Na-Cl-bearing fluids of a near-neutral pH composition. The corresponding depositional temperature is estimated at below 300 ◦C.

Keywords: Sasa Pb-Zn-Ag deposit; skarn; magmatic-hydrothermal ore deposits; ﬂuid inclusions; stable isotopes; EBSD; Serbo-Macedonian massif; postcollisional magmatism

1. Introduction Although skarn deposits represent products of interaction of a silicate melt (proximal skarns) or magmatic fluids (distal skarns) with a carbonate rich lithology, hydrothermal fluids play a significant role in evolution of all types of skarn deposits. Late hydrothermal overprints particularly affect the skarn deposit geometry, type of alteration products and ore distribution [1–4]. The Sasa Pb-Zn-Ag deposit is a typical distal skarn deposit and has been selected as a site to study processes that involve transport of base metals by magmatic and hydrothermal fluids as well as physicochemical factors that control the deposition of base metal-bearing mineral phases. The Sasa deposit (42.0◦ N, 22.5◦ E) is located on the Balkan Peninsula, approximately 150 km east from Skopje, Republic of Macedonia (Figure1). It consists of three ore-bearing localities: Svinja Reka, Golema Reka and Kozja Reka (Figure2a). The deposit hosts approximately 23.4 million metric tons of ore at 7.5% of Pb and Zn and up to 22 g/t Ag. Mining activities in the area date back to ancient times. The first geological investigations began in the 19th century and industrial production started in 1966. Since November 2017, the deposit is operated by Central Asia Metals. The Sasa Pb-Zn-Ag skarn deposit is hosted by the Serbo-Macedonian massif, a large elongate basement complex situated along the eastern part of the Balkan Peninsula. It extends southward from Serbia through Kosovo, Macedonia and Bulgaria to the Chalkidiki Peninsula in northern Greece (Figure1) and holds numerous economically important ore deposits of Cu, Au, Pb and Zn (e.g., Bor and Majdanpek, Serbia; Toranica, Sasa and Bucim, Macedonia; Osogovo, Bulgaria; Skouries, Greece [5–9]). The Sasa deposit is spatially and temporally associated with the Tertiary calc-alkaline magmatism [10]. It comprises prograde and retrograde mineral assemblages hosted by a sequence of Paleozoic marbles intercalated with quartz–graphite schists [11]. The prograde mineralization is represented by anhydrous Ca-Fe-Mn-silicate minerals (pyroxenes and pyroxenoids). A subsequent retrograde stage contains amphiboles, epidote, chlorites and ilvaite [12]. The principal ore minerals, galena and sphalerite, are accompanied by variable amounts of hydrothermal quartz and carbonates. This study presents the mineral chemistry, fluid inclusion and stable isotope data obtained on the skarn and hydrothermal mineral assemblages to give an insight into the evolution of the mineralizing fluids, to constrain the physiochemical conditions during the skarn formation and ore deposition and to refine the metallogenic model of the deposit. Geosciences 2018, 8, 444 3 of 28

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Figure 1. Regional geologic setting of the Sasa Pb-Zn-Ag skarn deposit, Republic of Macedonia, within Figure 1. Regional geologic setting of the Sasa Pb-Zn-Ag skarn deposit, Republic of Macedonia, the Balkan Peninsula (according to [13,14]). The locations of the most prominent Pb-Zn ± Ag within the Balkan Peninsula (according to [13,14]). The locations of the most prominent Pb-Zn ± Ag hydrothermal and Cu-Au porphyry deposits are also marked. Abbreviations: RKB = Ridanj–Krepoljin hydrothermal and Cu-Au porphyry deposits are also marked. Abbreviations: RKB = Ridanj–Krepoljin belt, TMC = Timok magmatic complex. belt, TMC = Timok magmatic complex. 2. Geological Setting 2. Geological Setting 2.1. Regional Geology 2.1. Regional Geology The Serbo-Macedonian massif is a N–S trending crystalline belt outcropping between the Vardar Theophiolite Serbo-Macedonian zone in the west massif and theis a RhodopeN–S trending massif crystalline in the east belt (Figure outcropping 1). This between Precambrian the Vardarto ophiolitePaleozoic zone terrain in the consists west andof two the main Rhodope tectonostratigraphic massif in the units, east the (Figure Lower unit1). This(also known Precambrian as the to PaleozoicLower terrain complex consists in Serbia of and two Macedonia, main tectonostratigraphic the Ograzhden unit units, in Bulgaria the Lower and the unit Vertiskos (also known unit in as the LowerGreece) complex and the in Upper Serbia unit and (the Macedonia, Vlasina unit the in Serbia Ograzhden and Macedonia unit in or Bulgaria the Morava and unit the in VertiskosBulgaria), unit in Greece)usually and distinguished the Upper by unit their (the metamorphic Vlasina unit grade. in Serbia The Lower and unit Macedonia was metamorphosed or the Morava up unit to in Bulgaria),medium usually- to lower distinguished-amphibolite byfacies their metamorphism, metamorphic whereas grade. the The Upper Lower unit unitunderwent was metamorphosed greenschist up tofacies medium- conditions to lower-amphibolite [13–16]. The Lower facies unit is metamorphism, composed predominately whereas the of gneisses, Upper unit micaschists, underwent quartzites, amphibolites, and occasionally marbles and migmatites. This unit has been considered as greenschist facies conditions [13–16]. The Lower unit is composed predominately of gneisses, a metamorphosed volcano–sedimentary sequence formed in the late Neoproterozoic to the earliest micaschists,Cambrian quartzites, along the amphibolites, active margin and of north occasionally Gondwana marbles that underwent and migmatites. the amphibolite This unit facies has been consideredmetamorphism as a metamorphosed during the Variscan volcano–sedimentary orogeny [13,17,18 sequence]. In contrast, formed the in the Upper late unit Neoproterozoic is mostly to the earliestcomposed Cambrian of the late along Neoproterozoic the active ocean margin floor of northsediments Gondwana and igneous that rocks, underwent overlaid the by a amphibolite Lower faciesOrdovician metamorphism to Lower during Carboniferous the Variscan sedimentary orogeny [ sequence13,17,18 ]. metamorphosed In contrast, the to Upper various unit schists, is mostly composedphyllites, of thequartzites late Neoproterozoic and marbles [13,19,20 ocean]. ﬂoor sediments and igneous rocks, overlaid by a Lower Ordovician to Lower Carboniferous sedimentary sequence metamorphosed to various schists, phyllites, quartzites and marbles [13,19,20]. Geosciences 2018, 8, 444 4 of 28

In the Cretaceous to Tertiary period, the Serbo-Macedonian massif as well as the adjunct Vardar zone were affected by considerable magmatic activity related to the Alpine orogeny and post-collisionalGeosciences 2018 collapse, 8, x FOR PEER of theREVIEW Alpine orogen, followed by extension of the Pannonian4 (Miocene) of 28 and AegeanIn the areas Cretaceous (Eocene–Pliocene) to Tertiary period, [21–24 the]. TheSerbo magmatism-Macedonian hadmassif an as intermediate, well as the adjunct mostly Vardar andesitic to trachytic,zone were character affected by and considerable resulted magmatic in the formation activity related of numerous to the Alpine ore orogeny deposits and including post-collisional porphyry Cu-Mo-Aucollapse and of the subordinate Alpine orogen epithermal, followed gold by depositsextension(e.g., of the Bor, Pannonian Majdanpek (Miocene) and Veliki and Aegean Krivelj areas in Serbia; Buchim(Eocene and– BorovPliocene) Dol [21 in– Macedonia;24]. The magmatism Skouries [ had8,9, 25 an– 28 intermediate,]) and Pb-Zn-Ag mostly hydrothermal andesitic to trachytic, deposits (e.g., Srebrenicacharacter in and Bosnia resulted and in Herzegovina; the formation of Cer numerous and Boranja ore deposits in Serbia; including Trepca, porphyry Crnac Cu and-Mo Belo-Au and Brdo in Kosovo;subordinate Sasa, Toranica epithermal and gold Zletovo deposits in Macedonia; (e.g., Bor, Majdanpek Olympias and in Greece;Veliki Krivelj [29–35 in], Serbia; Figure Buchim1a). The and age of magmatismBorov Dol decreases in Macedonia; from approximatelySkouries [8,9,25– 8428]) Ma and in Pb the-Zn north-Ag hydrothermal to 19 Ma in the deposits southernmost (e.g., Srebrenica part of the in Bosnia and Herzegovina; Cer and Boranja in Serbia; Trepca, Crnac and Belo Brdo in Kosovo; Sasa, Serbo-Macedonian massif [36,37]. Toranica and Zletovo in Macedonia; Olympias in Greece; [29–35], Figure 1a). The age of magmatism 2.2. Geologydecreases of thefrom Deposit approximately 84 Ma in the north to 19 Ma in the southernmost part of the Serbo- Macedonian massif [36,37]. The Sasa Pb-Zn-Ag skarn deposit is hosted by a Paleozoic metamorphic complex composed of marble2.2. Geology horizons of the intercalated Deposit with quartz–graphite schist and surrounded by Precambrian gneiss (Figure2).The The Sasa cross-section Pb-Zn-Ag skarn through deposit the is Pb-Zn-Ag hosted by Sasa a Paleozoic skarn depositmetamorphic reflects complex the strong composed lithological of controlmarble on ore horizons deposition intercalated revealing with that quartz carbonate–graphite rich schist lithology, and surrounded i.e. marble, by wasPrecambrian almost completely gneiss replaced(Figure by the2). The mineralization cross-section whereasthrough the other Pb-Zn country-Ag Sasa rocks, skarn including deposit reflects schists, the gneisses strong lithological and magmatic rocks,control are mostly on ore barrendeposition (Figure revealing2). The that mineralized carbonate rich strata lithology, dip ati.e. approximately marble, was almost 35 ◦ completelyto south-west and rangereplaced in thickness by the mineralization from 2 m to whereas 30 m. Rare other lenses country of preservedrocks, including marble schists, are characterized gneisses and by a magmatic rocks, are mostly barren (Figure 2). The mineralized strata dip at approximately 35° to uniform calcite grain size and fine intercalations of grey mica, classifying this marble to the cipollino south-west and range in thickness from 2 m to 30 m. Rare lenses of preserved marble are marble variety (Figure3a). The dark, medium grained and foliated quartz–graphite schist consists characterized by a uniform calcite grain size and fine intercalations of grey mica, classifying this predominatelymarble to the of cipollino graphite, marble quartz variety and (Figure minor 3a). sericite The dark, (Figure medium3b,c). grained The Precambrian and foliated quartz gneiss–graphite is strongly foliated,schist with consists ductile predominately deformed chlorite of graphite, and quartzquartz grainsand minor and sericite brittle (Figure deformed 3b,c amphiboles). The Precambrian (Figure 3d). The mineralizationgneiss is strongly is foliated, spatially with associated ductile deformed with magmatic chlorite and rocks quartz (Figure grains2), and mostly brittle of deformed trachytic to trachydaciticamphiboles composition (Figure 3d). The (Figure mineralization3e,f) and theis spatially K/Ar associated age is between with magmatic 31 Ma androcks 24(Figure Ma [2),38 ,39]. A relativelymostly of high trachytic 87 Sr/86 to trachydacit Sr ratioic (0.7095–0.7113, composition (Figure [38]) 3e, suggestsf) and the a significantK/Ar age is between crustal contamination31 Ma and common24 Ma for [38,39 the]. Oligocene–Miocene A relatively high 87 Sr/86 calc-alkaline Sr ratio (0.7095 to shoshonitic–0.7113, [38]) post-collisional suggests a significant magmatism crustal of the Balkancontamination Peninsula [ common40,41]. Although for the Oligocene the mineralization–Miocene calc is-alkaline lithologically to shoshonitic controlled, post- oldcollisional structures, magmatism of the Balkan Peninsula [40,41]. Although the mineralization is lithologically controlled, reactivated during the Tertiary post-collisional extension, might have played a significant role in old structures, reactivated during the Tertiary post-collisional extension, might have played a emplacement of magmatic bodies (Figure2). significant role in emplacement of magmatic bodies (Figure 2).

FigureFigure 2. (a )2. Geologic (a) Geologic map map of of the the Sasa Sasa Pb-Zn-Ag Pb-Zn-Ag skarn deposit deposit area; area; (b ()b Longitudinal) Longitudinal section section through through the Svinja Reka locality (from [39]). Abbreviation: AMSL — above mean sea level. the Svinja Reka locality (from [39]). Abbreviation: AMSL — above mean sea level.

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The mineralization consists of skarn and hydrothermal parageneses. The skarn parageneses are The mineralization consists of skarn and hydrothermal parageneses. The skarn parageneses are characterized by the presence of Mn-enriched calc-silicate minerals, including pyroxenes, characterized by the presence of Mn-enriched calc-silicate minerals, including pyroxenes, pyroxenoids, pyroxenoids, garnets, epidote, chlorites and ilvaite. The hydrothermal parageneses are mostly garnets, epidote, chlorites and ilvaite. The hydrothermal parageneses are mostly superimposed superimposed onto the skarn assemblages, and contain galena, sphalerite and pyrite, as well as minor onto the skarn assemblages, and contain galena, sphalerite and pyrite, as well as minor pyrrhotite, pyrrhotite, chalcopyrite and magnetite. Carbonates and quartz are the most abundant gangue chalcopyrite and magnetite. Carbonates and quartz are the most abundant gangue minerals. minerals.

FigureFigure 3. 3. (a(a) ) Cross Cross-cut-cut section section of of a a core core drilled drilled through through cipollino cipollino marb marble;le; (b) ( b Hand) Hand specimen specimen of ofquartz quartz–graphite–graphite schist schist;; (c) ( c Transmitted) Transmitted light light photomicrograph photomicrograph of of quartz quartz–graphite–graphite schist (cross (cross polars);polars); ( (dd)) Transmitted Transmitted light light photomicrograph photomicrograph of of Precambrian Precambrian gneiss gneiss (plane (plane polarized polarized light); light); (e) (Transmittede) Transmitted light light photomicrograph photomicrograph of of trachyte trachyte associated associated with with the the Sasa Sasa Pb-Zn-AgPb-Zn-Ag mineralization mineralization (plane(plane polarizedpolarized light);light); ((ff)) TransmittedTransmitted lightlight photomicrographphotomicrograph ofof trachytetrachyte associatedassociated withwith thethe SasaSasa Pb-Zn-AgPb-Zn-Ag mineralizationmineralization (crossed(crossed polars).polars). Abbreviations: Cal Cal—calcite;—calcite; Mc Mc—mica;—mica; Qtz Qtz—quartz;—quartz; Gr—graphite;Gr—graphite; Amp—amphiboles; Amp—amphiboles; Chl—chlorites; Chl—chlorites; Py—pyrite; Py—pyrite; Cpx—clinopyroxene; Cpx—clinopyroxene; Bt—biotite; Bt—biotite; Ms—muscovite;Ms—muscovite; Afs—alkaliAfs—alkali feldspars.feldspars.

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3. Materials and Methods A total of 30 samples were collected from the Svinja Reka locality, an active underground mine at the Sasa deposit. The representative samples of host rocks, associated magmatic rocks and ore mineralization were selected for further mineralogical and geochemical studies. Paragenetic relationships were studied in thin polished sections by transmitted and reflected polarized light microscopy. The X-ray powder diffraction (XRD) was conducted at the University of Zagreb on a Philips PW 3040/60 X’Pert PRO powder diffractometer (45 kV, 40 µA), with CuKα-monochromatized radiation (λ = 1.54056 Å) and θ–θ geometry. The area between 4◦ and 63◦ 2θ, with 0.02◦ steps, was measured with a 0.5◦ primary beam divergence. Compound identifications were based on a computer program X’ Pert high score of 1.0 B and literature data. The textural features and semi-quantitative analyses of mineralized samples were examined by a Zeiss Merlin Compact VP field emission Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-Ray (EDX) spectrometer and an Electron Backscattered Diffraction (EBSD) detector at UiT The Arctic University of Norway. EDX analyses were conducted with an X-Max80 EDX detector by Oxford instruments at a working distance of 8.5 mm, using an accelerating voltage of 20 kV and an aperture of 60 µm. The samples were mechanically polished and carbon-coated. The retrieved data were further processed by applying the AZtec software also provided by Oxford instruments. EBSD analyses for phase identification and distribution were conducted on a Nordlys EBSD detector in combination with the Aztec data processing software, both provided by Oxford instruments. The analyzed samples were mechanically and chemically polished with a colloidal silica solution and coated with a carbon layer. The samples were tilted to 70◦. An acceleration voltage of 20 kV was applied in combination with a 240 µm aperture. Step sizes for EBSD mapping were from 4.5 µm to 6 µm; six bands were detected with refined accuracy as indexing mode. Indexing rates were from 74.0% to 88.4%. A camera exposure time of 21 ms was applied in both cases. Petrographic and microthermometric measurements of fluid inclusions within transparent minerals (quartz, calcite, sphalerite and pyroxene) were performed at the University of Zagreb and at UiT The Arctic University of Norway. Double polished, 0.1 mm to 0.3 mm thick, transparent mineral wafers were studied. Measurements were carried out on Linkam THMS 600 stages mounted on an Olympus BX 51 (University of Zagreb) and an Olympus BX 2 (UiT) using 10× and 50× Olympus long-working distance objectives. Two synthetic fluid inclusion standards (SYN FLINC; pure H2O and ◦ mixed H2O–CO2) were used to calibrate the equipment. The precision of the system was ±2.0 C for homogenization temperatures, and ± 0.2 ◦C in the temperature range between −60◦ and +10 ◦C. Microthermometric measurements were made on carefully defined fluid inclusion assemblages, representing groups of inclusions that were trapped simultaneously. The fluid inclusion assemblages were identified based on petrography prior to heating and freezing. If all of the fluid inclusions within the assemblage showed similar homogenization temperature, the inclusions were assumed to have trapped the same fluid and to have not been modified by leakage or necking; these fluid inclusions thus record the original trapping conditions [42–44]. Carbon and oxygen isotope analyses of calcite separated from the host marble, as well as from skarn and hydrothermal mineral associations, were performed at the University of Lausanne and at UiT The Arctic University of Norway. In both laboratories, calcite powder was extracted from hand-picked samples using a dentist’s drill. A mass of 250 µg of powder was loaded in sealed reaction vessels, then flushed with helium gas and reacted at 72 ◦C with phosphoric acid. The evolved carbon dioxide was sampled using a ThermoFinnigan Gas-Bench and isotope ratios were measured in continuous flow mode using a ThermoFinnigan Delta + XP mass spectrometer. Data was extracted to an EXCEL file by using the ISODAT NT EXCEL export utility and further calculation steps were carried out using a predefined EXCEL Worksheet. Linearity corrections were applied based on the relationships between the intensity of the first sample peak (m/z 44) and δ 18O value of the standards. Due to calibration based directly on standard, which were part of each run (Carrara marble), correction for calcite runs was unnecessary. The stable carbon and oxygen isotope ratios are reported in the delta (notation as Geosciences 2018, 8, 444 7 of 28

Geosciences 2018, 8, x FOR PEER REVIEW 7 of 28 per mil ( ) deviation relative to the Vienna Standard Mean Ocean Water (V-SMOW) for oxygen and Vienna(V-SMOW) Peeh Dee for Belemnite oxygen and (V-PDB) Vienna for carbon. Pee Dee The Belemnite analytical reproducibility (V-PDB) for carbon.was better The than analytical±0.05 13 18 reproducibilityfor δ C and ± was0.1 betterfor δ thanO. ±0.05‰ for δ 13C and ± 0.1‰ for δ 18O. h h 4. Results 4.1. Petrography 4.1. Petrography The paragenetic sequence of the Sasa Pb-Zn-Ag skarn deposit (Figure4) illustrates that the The paragenetic sequence of the Sasa Pb-Zn-Ag skarn deposit (Figure 4) illustrates that the mineralization was deposited as a result of several mineralizing events, similar to other skarn deposits mineralization was deposited as a result of several mineralizing events, similar to other skarn worldwide [45–47]. Marble layers represent the main immediate host rock, and they are usually deposits worldwide [45–47]. Marble layers represent the main immediate host rock, and they are completely replaced by the mineralization. Textural features of the skarn parageneses, including usually completely replaced by the mineralization. Textural features of the skarn parageneses, rhythmic banding, scalloping and fingering, reflect infiltration-driven replacement as the main including rhythmic banding, scalloping and fingering, reflect infiltration-driven replacement as the mechanism of their formation (Figure5,[ 48]). The prograde stage has an anhydrous character with main mechanism of their formation (Figure 5, [48]). The prograde stage has an anhydrous character prevailing Ca-Fe-Mn pyroxenes and minor pyroxenoids and garnets. Pyroxenes form fibroradial with prevailing Ca-Fe-Mn pyroxenes and minor pyroxenoids and garnets. Pyroxenes form aggregates (Figure5). The retrograde stage resulted in a complex mineral assemblage that consists fibroradial aggregates (Figure 5). The retrograde stage resulted in a complex mineral assemblage that of a mixture of amphiboles, ilvaite, epidote, chlorites, magnetite, pyrrhotite, carbonates and quartz. consists of a mixture of amphiboles, ilvaite, epidote, chlorites, magnetite, pyrrhotite, carbonates and It texturally mimics the fibroradial texture inherited from the prograde mineralization (Figure5). quartz. It texturally mimics the fibroradial texture inherited from the prograde mineralization The superimposed hydrothermal mineral assemblages predominantly occur as replacements and (Figure 5). The superimposed hydrothermal mineral assemblages predominantly occur as open-space fillings. They contain galena, sphalerite, pyrite and minor chalcopyrite, as well as syn-ore replacements and open-space fillings. They contain galena, sphalerite, pyrite and minor chalcopyrite, and post-ore gangue carbonates and quartz (Figure6). as well as syn-ore and post-ore gangue carbonates and quartz (Figure 6).

Figure 4. Paragenetic sequence of the Sasa Pb Pb-Zn-Ag-Zn-Ag skar skarnn deposit.

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Figure 5. 5.( a()a Hand-specimen) Hand-specimen showing showing the transition the transition from the from prograde the prograde (anhydrous (anhydrous silicate-dominated) silicate- dominated)stage to the retrogradestage to the (hydrous retrograde silicate- (hydrous dominated) silicate- stage;dominated) (b) Hand-specimen stage; (b) Hand showing-specimen the rhythmicshowing thebedding rhythmic of the bedding retrograde/hydrothermal of the retrograde/hydrothermal mineral paragenesis; mineral (c) Transmitted paragenesis; light (c) Transmitted photomicrograph light photomicrographof the retrograde of mineralization the retrograde mimicking mineralization prograde mimicking ﬁbroradial prograde texture fibroradial (plane polarized texture (plane light); (polarizedd) Transmitted light); light (d) Transmitted photomicrograph light ofphotomicrograph the retrograde mineralization of the retrograde mimicking mineralization prograde mimicking ﬁbroradial progradetexture (crossed fibroradia polars);l texture (e) Transmitted (crossed polars); light photomicrograph (e) Transmitted oflight prograde photomicrograph pyroxene partly of prograde replaced pyroxeneby pyrrhotite partly (plane replaced polarized by pyrrhotite light); ((planef) Transmitted polarized light light); photomicrograph (f) Transmitted light of prograde photomicrograph pyroxene partlyof prograde replaced pyroxene by pyrrhotite partly replaced (crossed by polars). pyrrhotite Abbreviations: (crossed polars). Px—pyroxene; Abbreviations: Amph—Amphibole; Px—pyroxene; AmphChl—chlorite;—Amphibole; Sph—Sphalerite; Chl—chlorite; Ep—epidote; Sph—Sphalerite; Po—pyrrhotite; Ep—epidote; Py—Pyrite. Po—pyrrhotite; Py—Pyrite.

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Figure 6. (a) The rhythmic banding of the prograde mineralization with the retrograde and Figure 6. (a) The rhythmic banding of the prograde mineralization with the retrograde and hydrothermal mineral parageneses suggests that replacement processes were important mechanisms hydrothermal mineral parageneses suggests that replacement processes were important mechanisms for for the development of retrograde alterations, as well as for deposition of the hydrothermal the development of retrograde alterations, as well as for deposition of the hydrothermal mineralization; mineralization; (b) Hand-specimen showing a typical hydrothermal paragenesis composed of (b) Hand-specimenpyrrhotite, chalcopyrite, showing sphalerite a typical a hydrothermalnd quartz; (c) Hand paragenesis-specimen composed consisting of of pyrrhotite, pyrite, galena chalcopyrite, and sphaleritecarbonates and reflects quartz; the (c open) Hand-specimen-space deposition consisting of the hydrothermal of pyrite, mineralization; galena and carbonates (d) Post-ore b reflectsladed the open-spacecalcite; deposition(e) Reflected of the light hydrothermal photomicrograph mineralization; of hydrothermal (d) Post-ore mineral bladed paragenesis calcite; consisting(e) Reflected of light photomicrographmagnetite, chalcopyrite, of hydrothermal galena mineral and sphalerite; paragenesis (f) Reflected consisting light of magnetite, photomicrograph chalcopyrite, showing galena and sphalerite;chalcopyrite (f disease) Reflected in Fe light-rich photomicrographsphalerite. This texture showing is often chalcopyrite interpreted diseaseas a diffusion in Fe-rich-controlled sphalerite. This texturereplacement is often of the interpreted Fe-rich sphalerite as a diffusion-controlled mostly along crysta replacementl planes or controlledof the Fe-rich by compositional sphalerite mostly alongvariabilities crystal planes within or the controlled sphalerite. by Abbre compositionalviations: Px variabilities—pyroxene; Amph within— theAmphibole; sphalerite. Chl Abbreviations:—chlorite; Px—pyroxene;Gn—Galena; Amph—Amphibole; Sph—Sphalerite; Cpy Chl—chlorite;—Chalcopyrite; Gn—Galena; Po—Pyrrhotite; Sph—Sphalerite; Mt—Magnetite; Cpy—Chalcopyrite; Qtz—Quartz; Carb—Carbonates. Po—Pyrrhotite; Mt—Magnetite; Qtz—Quartz; Carb—Carbonates.

4.2. X-ray4.2. X Diffraction-ray Diffraction (XRD) (XRD) Hydrothermally altered skarn assemblages are characterized by very fine-grained textures that Hydrothermally altered skarn assemblages are characterized by very ﬁne-grained textures preclude determination of their mineral composition by optical microscopy techniques and, that preclude determination of their mineral composition by optical microscopy techniques and, alternatively, the XRD method was applied. The representative XRD patterns presented in Figure 7 alternatively, the XRD method was applied. The representative XRD patterns presented in Figure7 suggest that prograde mineral assemblages composed mostly of pyroxenes from the hedenbergite–johannsenite series, which are altered to amphiboles from the actinolite–ferroactinolite Geosciences 2018, 8, x FOR PEER REVIEW 10 of 28

Geosciences 2018, 8, 444 10 of 28 suggest that prograde mineral assemblages composed mostly of pyroxenes from the hedenbergite–johannsenite series, which are altered to amphiboles from the actinolite–ferroactinolite series, serpentine, minerals minerals from from the epidote group, chlorites of the clinochlore type, magnetite, carbonates and and quartz. quartz. In addition, In addition, ore minerals, ore minerals, including including pyrite, galena, pyrite, sphalerite galena, and sphalerite chalcopyrite, and chalcopyrite,have been recorded. have been recorded.

Figure 7. Representative X X-ray-ray Diffraction ( (XRD)XRD) patterns reveal a complexcomplex mineral composition of retrograde alterations: alterations: (a )( Progradea) Prograde pyroxene pyroxene (hedenbergite) (hedenbergite partly) altered partly to altered a mixture to of a ferroactinolite mixture of ferroactinoliteand serpentine and overprinted serpentine and by the overprinted hydrothermal by themineralization hydrothermal composed mineralization of galena, composed chalcopyrite of galena,and quartz; chalcopyrite (b) Completely and quartz; altered (b) progradeCompletely mineralization altered prograde into amineralization mixture of epidote, into a piemontitemixture of epidote,and serpentine. piemontite Hydrothermal and serpentine. pyrite Hydrothermal and quartz arepyrite also and present; quartz ( care) Completely also present; altered (c) Completely prograde alteredmineralization prograde into mineralization a mixture of ferroactinolite, into a mixture actinolite,of ferroactinolite, magnetite, actinolite, carbonate magnetite, and quartz. carbonat Galenae and quartz.sphalerite Galena have and been sphalerite recorded have as well. been recorded as well.

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Geosciences 2018, 8, x FOR PEER REVIEW 11 of 28 4.3. Electron Back Scattered Diffraction (EBSD) 4.3. Electron Back Scattered Diffraction (EBSD) The EBSDThe EBSD method method was was utilized utilized to to determine determine the spatial spatial distribution distribution of ofretrograde retrograde alteration alteration products.products. The The EBSD EBSD maps maps presented presented in in Figures Figures8 8 and and9 9 suggest suggest that that prograde prograde pyroxenes pyroxenes have have been been alteredaltered to various to various degrees degrees during during the the infiltration infiltration of of hydrothermal hydrothermal fluids. fluids. Sharp Sharp alteration alteration zones zones composedcomposed of a mixture of a mixture of hydrous of hydrous calc-silicate calc-silicate minerals, minerals, predominantly predominantly amphiboles, amphiboles, chlorites, chlorites, epidote and ilvaiteepidote pointand ilvaite to an point increase to an inincrease water in activity water activity (Figure (Figure8). In 8). contrast, In contrast, mixtures mixtures of of quartz quartz and carbonatesand car usuallybonates overprintusually overprint individual individual pyroxene pyroxene grains, grains, mimicking mimicking the progradethe prograde fibroradial fibroradial texture texture and reflecting an increase in the CO2 fugacity (Figure 9). Carbonates from both the calcite and and reflecting an increase in the CO2 fugacity (Figure9). Carbonates from both the calcite and dolomite dolomite groups have been identified. Fine grained sulfide minerals suggest an increase in the sulfur groups have been identified. Fine grained sulfide minerals suggest an increase in the sulfur fugacity. fugacity.

Figure 8. The Electron Back Scattered Diffraction (EBSD) maps illustrating the retrograde alterations Figure 8. The Electron Back Scattered Diffraction (EBSD) maps illustrating the retrograde alterations of prograde pyroxene under high water activity: (a) Band contrast EBSD map; (b) Distribution of of prograde pyroxene under high water activity: (a) Band contrast EBSD map; (b) Distribution of prograde clinopyroxene and retrograde hydrous silicates; (c) Distribution of prograde clinopyroxene prograde clinopyroxene and retrograde hydrous silicates; (c) Distribution of prograde clinopyroxene and retrograde/hydrothermal carbonates and quartz; (d) Distribution of prograde clinopyroxene and and retrograde/hydrothermalhydrothermal sulfide mineralization. carbonates The and EBSD quartz; mapping (d) Distributionstep size was 4.5 of progradeµ m and the clinopyroxene indexing rate and hydrothermalwas 74.0%. sulﬁde mineralization. The EBSD mapping step size was 4.5 µm and the indexing rate was 74.0%.

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Figure 9. TheThe EB EBSDSD maps illustrating the retrograde alterations of prograde pyroxene under an increased CO2 fugacity: (a) Band contrast EBSD map; (b) Distribution of prograde clinopyroxene and increased CO2 fugacity: (a) Band contrast EBSD map; (b) Distribution of prograde clinopyroxene and retrograde hydrous hydrous silicates; silicates; (c) Distribution (c) of Distribution prograde clinopyroxene of prograde and retrograde/hydrothermal clinopyroxene and carbonatesretrograde/hydrothermal and quartz; ( carbonatesd) Distribution and quartz; of prograde (d) Distribution clinopyroxene of prograde and hydrothermal clinopyroxene sulfide and mineralization.hydrothermal sulfide The EBSD mineralization. mapping step The size EBSD was mapping 6 µm and step the indexingsize was rate6 µ m was and 88.4%. the indexing rate was 88.4%. 4.4. Mineral Chemistry 4.4. Mineral Chemistry The major element composition of pyroxenes, analyzed by the EDS/SEM technique, is listed in TableThe1 major and plottedelement incomposition the diopside–johannsenite–hedenbergite of pyroxenes, analyzed by the EDS/SEM ternary diagramtechnique, (Figure is listed 10 in). Hedenbergite,Table 1 and withplotted 74–80 in mol the %, diopside represents–johannsenite the main pyroxene–hedenbergite constituent. ternary The diagram johannsenite (Figure content 10). variesHedenbergite, between with 16–21 74 mol–80 mol % and %, diopsiderepresents between the main 1–6 pyroxene mol %. constituent. The johannsenite content variesThe between EDS/SEM 16–21 analyses mol % and of sulfides diopside revealed between that 1– sphalerite6 mol %. owes its black color to an enrichment of ironThe (>10 EDS/SEM wt. % of analyses Fe, Table of2). sulfides Sphalerite revealed also contains that sphaleri detectablete owes amounts its black of color cadmium, to an manganeseenrichment andof iron copper (>10 wt (Table. % of2). Fe, Galena Table carries2). Sphalerite significant also contains amounts detectable of bismuth amounts (up to 4.7of cadmium, wt. % of Bi). manganese Indium and silvercopper have (Table been 2). detectedGalena carries as well significant (Table2). Variationsamounts of in bismuth the trace (up element to 4.7 contentwt. % of of Bi). sphalerite Indium and galenasilver ha areve presented been detected in Figure as well 11. (Table 2). Variations in the trace element content of sphalerite and galena are presented in Figure 11.

Table 1. Chemical composition of pyroxenes from the Sasa Pb-Zn-Ag skarn deposit. Hd— hedenbergite; Jo—johannsenite; Di—diopside.

Sample Point CaO FeO MnO MgO SiO2 Hd Jo Di wt. % % SA-101 Px-1 24.6 22.2 5.1 0.5 47.6 79.8 18.4 1.8 Px-2 25.1 20.5 4.3 1.1 49.0 79.1 16.5 4.4 Px-3 24.4 21.8 4.7 0.8 48.3 79.8 17.2 3.0 Px-4 22.8 21.2 5.6 1.0 49.4 76.3 20.1 3.7 Px-5 25.0 21.2 5.7 0.7 47.5 77.0 20.6 2.4

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Table 1. Chemical composition of pyroxenes from the Sasa Pb-Zn-Ag skarn deposit. Hd—hedenbergite; Jo—johannsenite; Di—diopside.

Sample Point CaO FeO MnO MgO SiO2 Hd Jo Di wt. % % SA-101 Px-1 24.6 22.2 5.1 0.5 47.6 79.8 18.4 1.8 Geosciences 2018, 8, x Px-2FOR PEER 25.1REVIEW 20.5 4.3 1.1 49.0 79.1 16.5 4.4 13 of 28 Px-3 24.4 21.8 4.7 0.8 48.3 79.8 17.2 3.0 Px-4 22.8 21.2 5.6 1.0 49.4 76.3 20.1 3.7 Px-6 24.0 22.8 5.1 0.7 47.4 79.7 17.9 2.4 Px-5 25.0 21.2 5.7 0.7 47.5 77.0 20.6 2.4 Px-6Px-7 24.023.2 22.819.7 5.14.9 0.70.8 47.451.5 79.777.5 17.919.5 2.43.0 Px-7Px-8 23.223.1 19.721.2 4.96.0 0.80.8 51.549.0 77.575.7 19.521.3 3.03.0 Px-8Px-9 23.123.9 21.220.2 6.05.2 0.81.0 49.049.6 75.776.4 21.319.8 3.03.8 Px-9Px-10 23.922.7 20.219.1 5.25.2 1.01.6 49.651.5 76.473.9 19.820.1 3.86.0 Px-10 22.7 19.1 5.2 1.6 51.5 73.9 20.1 6.0 SA-102 Px-11 25.2 21.1 5.2 0.6 47.9 78.6 19.2 2.1 SA-102 Px-11Px-12 25.225.4 21.119.8 5.25.3 0.60.7 47.948.8 78.676.8 19.220.5 2.12.8 Px-12 25.4 19.8 5.3 0.7 48.8 76.8 20.5 2.8 Px-13Px-13 25.425.4 18.918.9 5.35.3 0.70.7 49.649.6 75.775.7 21.421.4 2.92.9

Geosciences 2018, 8Px-14, x FORPx PEER-14 23.9 REVIEW23.9 21.221.2 5.65.6 0.40.4 48.848.8 77.977.9 20.420.4 1.61.614 of 28 Px-15Px-15 24.024.0 21.021.0 5.55.5 0.40.4 49.149.1 78.278.2 20.420.4 1.31.3 Px-16 24.6 20.4 4.6 0.9 49.4 78.9 17.8 3.4 Sasa-20 Px-Galena16 24.6 11 20.4 4.6 0.9 49.4 78.9 17.8 3.4 Px-17Px-17 23.423.4 20.3 20.3 4.64.6 1.61.6 50.150.1 76.576.5 17.517.5 5.95.9 Px-18 23.0 22.8Pb 5.3 1.083.12 48.084.75 78.4 83.96 18.1 0.59 3.4

Px-19Px-18 24.623.0 19.822.8 Bi 5.05.3 0.72.97 1.0 49.948.03.55 77.378.43.25 19.7 18.1 0.19 2.9 3.4

Px-20Px-19 22.524.6 19.8 19.8Ag 5.4 5.0 0.7

Mn 0.35 0.43 0.39 0.03 Figure 10. Jo-Di-Hd (Mn -Mg-Fe) ternaryCd diagram showing0.58 compositional0.63 variations0.61 of pyroxenes0.02 Figure 10. Jo-Di-Hd (Mn-Mg-Fe) ternary diagram showing compositional variations of pyroxenes from the Sasa Pb-Zn-Ag skarn deposit. EndCu members

Sample Mineralogy n Element Minimum Maximum Mean STD Sasa-17 Galena 7 wt.% Pb 77.76 82.26 80.23 1.52 Bi 4.02 4.69 4.36 0.23 Ag 0.02 0.62 0.16 0.21 In 0.15 0.31 0.21 0.06 S 12.12 12.57 12.37 0.18 Total 94.30 99.22 97.33 1.69 Sasa-17 Sphalerite 4 Zn 58.44 59.34 58.86 0.39

Fe 10.18 10.38 10.25 0.09 Figure 11. Variations in the trace element content of (a) Sphalerite; (b) Galena from the Figure 11. Variations in the trace element contentMn of (a)0.28 Sphalerite; (b)0.34 Galena from0.32 the Sasa0.03 Pb-Zn-Ag Sasa Pb-Zn-Ag skarn deposit. Cd 0.43 0.58 0.51 0.08 skarn deposit. Cu

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Table 2. Semi-quantitative composition of sulﬁdes from the Sasa Pb-Zn-Ag skarn deposit. n—number of point analyses per sample;

4.5. Fluid Inclusion Studies Fluid inclusions entrapped within skarn and hydrothermal minerals preserve information about mineralizing ﬂuids in a magmatic-hydrothermal system and reveal variations of pressure–temperature–chemical composition (P–T–X) conditions over time. Fluid inclusion data collected from pyroxenes, quartz and carbonates from the Sasa Pb-Zn-Ag skarn deposit are summarized in Table3. Geosciences 2018, 8, 444 15 of 28

Table 3. Summary of the ﬂuid inclusion data obtained from the Sasa Pb-Zn-Ag skarn deposit. Px—pyroxene; Qtz—quartz; Cc—calcite; F—Degree of ﬁll; L—Liquid phase; V—Vapor phase.

Mineralization Prograde Retrograde Hydrothermal Type Host Mineral Px Qtz Syn-ore Qtz Syn-ore Qtz Post-ore Cc Post-ore Cc Fluid Inclusion Primary Primary Primary Secondary Primary Secondary Type Phases (at 25 ◦C) L+V L+V L+V L+V L+V L+V F (at 25 ◦C) 0.6 0.7–0.8 0.7 0.8–0.9 0.7–0.8 0.9 NaCl-MgCl2- NaCl-CaCl2- NaCl-MgCl2- NaCl-CaCl2- NaCl-CaCl2- NaCl-CaCl2- Composition H2O or FeCl2- H2O H2O H2O H2O H2O H2O Salinity 7.5–9.6 9.3–10.9 3.9–9.1 4.2–6.3 4.8–8.8 2.2–2.9 (wt% NaCl equ.) Th (◦C) 403–433 237–390 240–394 125–239 242–297 130–145 Density (g/cm3) 0.5270.598 0.680–0.899 0.612–0.890 0.852–0.966 0.820–0.856 0.944–0.951

Primary fluid inclusions entrapped by pyroxene are usually found in isolated clusters or along growth zones. The inclusions come in a variety of shapes, mostly elongated, squared and irregular. Their size ranges from <2–20 µm. At room temperature, they contain liquid (L) and vapor (V) phases and are characterized by a uniform degree of fill around 0.6 (Figure 12a,b). The eutectic temperature ◦ (Te) recorded near −50 C reveals NaCl and CaCl2 as the principal dissolved salts [49]. The final ice melting temperature in the range between −6.3 ◦C and −4.7 ◦C corresponds to the salinity between 7.5–9.6 wt. % NaCl equ. [50]. Homogenization to liquid phase was obtained between 403–433 ◦C. The calculated bulk density spans from 0.527–0.598 g/cm3. Secondary inclusions occur along healed fractures. Their size of <2 µm precludes reliable microthermometric measurements. Quartz associated with retrogradely altered skarn assemblages hosts visible primary fluid inclusions. Primary fluid inclusions occur within clusters. They are mostly rounded or of irregular shape and range in size up to 20 µm. At room temperature, these types of inclusions contain liquid and vapor phases (Figure 12c). The eutectic temperature around −31 ◦C indicates that the mineralizing ◦ ◦ fluids were enriched in MgCl2 [49]. The final ice melting temperature between −7.5 C and −6.1 C corresponds to the apparent salinity between 9.3–10.9 wt. % NaCl equ. [50]. Homogenization to liquid phase was recorded in the wide temperature range from 237–390 ◦C. Syn-ore quartz entrapped primary fluid inclusions mostly along its growth zones. The inclusions are two-phase (L and V), have irregular shapes and their size varies up to 25 µm. The degree of fill is estimated around 0.7 (Figure 12d). The eutectic temperature around −35 ◦C suggests a ◦ MgCl2-NaCl-H2O or FeCl2-H2O system [49]. The final ice melting temperature between −2.3 C and −5.9 ◦C points to the apparent salinity between 3.9–9.1 wt. % NaCl equiv. The total homogenization by the vapor phase disappearance is recorded in the temperature interval between 240–394 ◦C. The calculated fluid density spans from 0.665–0.858 g/cm3. Secondary inclusion trails crosscut mineral grains and contain two-phase (L and V) inclusions of irregular shape with the degree of fill between 0.8 and 0.9 (Figure 12e). They entrapped moderate salinity fluids (4.2–6.3 wt. % NaCl equ.) and exhibit homogenization into the liquid phase between 125–239 ◦C. The post-ore stage of the mineral parageneses is characterized by deposition of a significant amount of carbonates, predominantly calcite, and a minor amount of quartz. Primary inclusions in post-ore carbonates commonly occur in isolated clusters and show negative crystal shapes. Their size ranges from 5–50 µm. They contain two phases (L and V) and have the degree of fill between 0.7 and 0.8 (Figure 12f). Obtained microthermometric data indicate that post-ore minerals were deposited ◦ from cooler CaCl2-NaCl-H2O solutions of moderate salinity (Te ≈ −52 C; salinity = 4.8–8.8 wt. % NaCl equiv; Th = 242−297 ◦C). Rare secondary inclusions revealed that diluted and relatively cold fluids (salinity = 2.2–2.9 wt. % NaCl equiv; Th = 130–145 ◦C) were circulating in the area even when the mineralizing processes had terminated. Geosciences 2018, 8, 444 16 of 28 Geosciences 2018, 8, x FOR PEER REVIEW 16 of 28

FigureFigure 12. 12.Photomicrographs Photomicrographs of of fluid fluid inclusions inclusions from from the the Sasa Sasa Pb-Zn-Ag Pb-Zn-Ag skarn skarn deposit; deposit; ( a(a)) Primary Primary two-phasetwo-phase (L (L and and V) V) fluid fluid inclusions inclusions hosted hosted along along growth growth zones zones of of hedenbergite; hedenbergite; ( b(b)) Isolated Isolated primary primary two-phasetwo-phase fluid fluid inclusions inclusions in in hedenbergite; hedenbergite; (c) ( Primaryc) Primary two-phase two-phase (L and(L and V) fluidV) fluid inclusions inclusions hosted hosted by retrogradeby retrograde quartz; quartz; (d) Primary (d) Primary fluid fluid inclusions inclusions hosted hosted by syn-ore by syn quartz;-ore quartz; (e) Trail (e) of Trail secondary of secondary fluid inclusionsfluid inclusions within within syn-ore syn quartz;-ore quartz; (f) Primary (f) Primary fluid inclusions fluid inclusions hosted hosted by post-ore by post calcite.-ore calcite.

4.6.4.6. Stable Stable Isotope Isotope Data Data 13 18 TheTheδ δ13CC andandδ δ18OO valuesvalues obtainedobtained fromfrom differentdifferent generationsgenerations ofof calcitecalcite areare listedlisted inin TableTable4 4and and 13 18 shownshown in in Figure Figure 13 13.. The The barren barren host host cipollino cipollino marble marble has hasδ δ13CC andandδ δ18OO valuesvalues ofof 1.41.4‰V-PDB V-PDB and and 13 26.326.3‰V-SMOW, V-SMOW, respectively. respectively. Calcite Calcite associated associated with with skarn skarn mineral mineral parageneses parageneses have havehδ δ13CC values values 18 betweenbetweenh − −7.47.4‰and and −−7.2‰ V-PDBV-PDB andand δδ18O values between 5.7 ‰ andand 7.07.0‰V-SMOW. V-SMOW. Syn-ore Syn-ore and and 13 18 post-orepost-ore hydrothermal hydrothermalh calcite calciteh exhibit exhibit mostly mostly overlapping overlappingδ δ13CC and andhδ δ18OO valuesvaluesh in in the the range range between between −−6.46.4‰and and− −4.14.1‰ V-PDBV-PDB andand 13.913.9‰and and 15.415.4‰V-SMOW, V-SMOW, respectively. respectively. h h h h

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Geosciences 2018, 8, 444 17 of 28 Table 4. Carbon and oxygen isotope composition of calcite from the Sasa Pb-Zn-Ag skarn deposit. V-SMOW = Vienna Standard Mean Ocean Water. V-PDB = Vienna Pee Dee Belemnite. Table 4. Carbon and oxygen isotope composition of calcite from the Sasa Pb-Zn-Ag skarn deposit. V-SMOW = Vienna Standard Mean Ocean Water. V-PDB13 = Vienna Pee Dee18 Belemnite. Sample Type of Mineralization δ C (‰. V-PDB) δ O (‰. V-SMOW) 13 18 SampleSa-1-C Type of MineralizationCippolino marble δ C( 1.4. V-PDB) δ26.3O( . V-SMOW) Sa-1-CSa-101 CippolinoAltered marble skarn h− 1.47.4 5.7 h 26.3 Sa-101Sa-101-1 AlteredAltered skarn skarn −−7.37.4 6.4 5.7 Sa-101-1Sa-102 AlteredAltered skarn skarn −−7.27.3 7.0 6.4 Sa-102Sa-103 AlteredAltered skarn skarn −−7.37.2 6.4 7.0 − Sa-103Sa-15 AlteredHydrothermal skarn ore −4.77.3 14.6 6.4 Sa-15 Hydrothermal ore −4.7 14.6 Sa-15-2 Hydrothermal ore −4.8 14.4 Sa-15-2 Hydrothermal ore −4.8 14.4 Sa-15-3 Hydrothermal ore −4.8 14.6 Sa-15-3 Hydrothermal ore −4.8 14.6 Sa-16-CSa-16-C HydrothermalHydrothermal ore ore −−5.15.1 14.7 14.7 Sa-17Sa-17 HydrothermalHydrothermal ore ore −−6.06.0 14.3 14.3 Sa-17-0Sa-17-0 HydrothermalHydrothermal ore ore −−5.65.6 15.4 15.4 Sa-17-1Sa-17-1 HydrothermalHydrothermal ore ore −−5.85.8 14.7 14.7 Sa-17-M1Sa-17-M1 HydrothermalHydrothermal ore ore −−4.14.1 13.9 13.9 Sa-17-M2Sa-17-M2 HydrothermalHydrothermal ore ore −−4.24.2 13.9 13.9 Sa-17-CSa-17-C HydrothermalHydrothermal ore ore −−5.65.6 14.7 14.7 − Sa-18-OSa-18-O HydrothermalHydrothermal ore ore −6.46.4 8.3 8.3 Sa-19 Hydrothermal ore −6.0 14.4 Sa-19 Hydrothermal ore −6.0 14.4 Sa-19-C Hydrothermal ore −5.0 14.8 Sa-19-C Hydrothermal ore −5.0 14.8

Figure 13.13. δ1313CC vs. δδ1818OO plot plot of of different different generation generationss of of carbonates from the Sasa Pb-Zn-AgPb-Zn-Ag skarn deposit. Reference values for marine carbonates reference values for magmatic carbonates from [[5511] and [[5522].]. 5. Discussion 5. Discussion Geological, mineralogical and geochemical features of the Sasa Pb-Zn-Ag deposit classify this Geological, mineralogical and geochemical features of the Sasa Pb-Zn-Ag deposit classify this deposit to the group of calcic Pb-Zn skarn deposits [45]. Although the mineralization is closely deposit to the group of calcic Pb-Zn skarn deposits [45]. Although the mineralization is closely associated with magmatic rocks, direct contacts between the mineralization and the magmatic rocks associated with magmatic rocks, direct contacts between the mineralization and the magmatic rocks are obscure (Figure2), suggesting a distal character of the deposit and the interaction of mineralizing are obscure (Figure 2), suggesting a distal character of the deposit and the interaction of mineralizing ﬂuids with the host carbonate rocks (cipollino marble) as the major mineralizing mechanism. fluids with the host carbonate rocks (cipollino marble) as the major mineralizing mechanism. Geochemical features (trachytic to trachydacitic composition; calc-alkaline character; Na2O/K2O Geochemical features (trachytic to trachydacitic composition; calc-alkaline character; Na2O/K2O < 1; high large-ion lithophile element to high ﬁeld strength element ratios (LILE/HFSE); strong < 1; high large-ion lithophile element to high field strength element ratios (LILE/HFSE); strong enrichment in K, Pb and U) as well as their K/Ar age (31–24 Ma) suggest that magmatic rocks enrichment in K, Pb and U) as well as their K/Ar age (31−24 Ma) suggest that magmatic rocks

Geosciences 2018, 8, x FOR PEER REVIEW 18 of 28 associated with the Sasa deposit are a product of the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene period [21,23,38–41,53], resulting in the formation of numerous magmatic–hydrothermal ore deposits along the Vardar Zone and the Serbo-Macedonian Massif (Figure 1; [25–35,54]). The paragenetic sequence (Figure 4) indicates that, during its formation, the Sasa Pb-Zn-Ag Geosciences 2018, 8, 444 18 of 28 deposit underwent three main stages similar to other known skarn deposits worldwide: (1) a stage of isochemical metamorphism; (2) an anhydrous prograde stage and (3) a retrograde/hydrothermal stage [1,55,56]. As the deposit is hosted by a highly metamorphosed terrain, it is difficult to distinguishassociated the regional with metamorphic the Sasa deposit signature are from a product the metamorphism of the calc-alkaline associated with to shoshoniticthe post-collisional emplacementmagmatism of Tertiary that magmatic affected bodies. the Balkan However, Peninsula the isotopic during composition the Oligocene–Mioceneof preserved lenses of period [21,23,38–41,53], the hostresulting marble overlap in the with formation values typical of numerousfor marine carbonates magmatic–hydrothermal, suggesting that the metamor ore depositsphism along the Vardar Zone has not disturbed its primary isotopic composition. Theand prograde the Serbo-Macedonian mineral assemblage Massif is marked (Figure by the1 ;[ presence25–35, 54 of ]). anhydrous Ca-Fe-Mg-Mn silicates, predominantlyThe paragenetic pyroxenes sequence from the hedenbergite (Figure4)–johannsenite indicates that,series. duringThe predomina its formation,nce of the Sasa Pb-Zn-Ag anhydrousdeposit mineral underwents reflects a low three water main activity stages, whereas similar the prevalence to other of known pyroxene skarn (hedenbergite) deposits worldwide: (1) a stage over garnetsof isochemical (andradite) suggest metamorphism;s a high ferrous/ferric (2) an ratio anhydrous and a relatively prograde reductive stage environment and (3) [57]. a retrograde/hydrothermal Absence of Fe-sulfides indicates a low sulfur fugacity during the prograde stage. Although our SEM/EDSstage analyses [1,55 ,56 (Table]. As 1) the revealed deposit that is pyroxenes hosted by are a characterized highly metamorphosed by a relatively terrain, high and it is difficult to distinguish uniformthe FeO/MnO regional ratio, metamorphic previously published signature data [12,58 from] suggest the metamorphism greater variations in associated the pyroxene with the emplacement of compositiTertiaryon among magmatic different bodies. ore bodies However, and local the predomination isotopic composition of Mn-rich pyroxenes.of preserved The lenses of the host marble variationsoverlap may be with controlled values by typical periodical for variations marine in carbonates, the chemistry suggesting of infiltrating that magmatic the metamorphism fluids: has not disturbed CaCOits3 (calcite) primary + 2 H isotopic4SiO4 + Fe composition.2+ ↔ CaFeSi2O6 (hedenbergite) + 3 H2O + CO2 + 2 H+ (1) The prograde mineral assemblage is marked by the presence of anhydrous Ca-Fe-Mg-Mn silicates, CaCO3 (calcite) + 2 H4SiO4 + Mn2+ ↔ CaMnSi2O6 (johansennite) + 3 H2O + CO2 + 2 H+ (1) predominantly pyroxenes from the hedenbergite–johannsenite series. The predominance of anhydrous and/or in local temperature oscillations: minerals reflects a low water activity, whereas the prevalence of pyroxene (hedenbergite) over garnets CaFeSi2O6 (hedenbergite) + Mn2+ ↔ CaMnSi2O6 (johannsenite) + Fe2+ (andradite) suggests a high ferrous/ferric ratio and a relatively reductive environment(2) [57]. Absence of Fe-sulfides indicates a lowrH25 sulfur °C, 1atm = fugacity−7 kJ/mole during * the prograde stage. Although our SEM/EDS analyses (Table* Calculated1) revealed for standard that pyroxenes conditions using are thermodynamic characterized data bypublished a relatively by [59,60] high. and uniform FeO/MnO ratio, Inpreviously contrast to the published carbonate data component [12,58 of] suggestthe host cipollino greater marble variations that was in replaced the pyroxene by composition among pyroxenes,different grey micaore bodies has not and been local significantly predomination affected by of metasomatic Mn-rich pyroxenes. processes during The variations the may be controlled progradeby stage periodical of the mineralization, variations in probably the chemistry due to the ofinsufficient infiltrating water magmatic activity and fluids: a relatively high K+/H+ molar ratio:

2 KAl2(AlSi3O10)(OH)2 (kaolinite) + 3 H2O + 2 2+H+ ↔ 3 Al2Si2O5(OH)4 (mica) + 2 K+ (3) + CaCO3 (calcite) + 2 H4SiO4 + Fe CaFeSi2O6 (hedenbergite) + 3 H2O + CO2 + 2 H (1) According to the fluid inclusion data, the prograde stage occurred at temperatures above 405 °C and at pressures above 30 MPa under the influence of2+ moderate salinity and low density Ca-Na- + CaCO3 (calcite) + 2 H4SiO4 + Mn CaMnSi2O6 (johansennite) + 3 H2O + CO2 + 2 H (2) chloride bearing aqueous fluids. The absence of liquid CO2 indicates that XCO2 was below 0.1 [61–63and/or]. Due to absence in local of temperature any reliable independent oscillations: geothermometer and/or geobarometer, only the minimum P-T conditions can be set (Figure 14). Textural relations (Figures 5, 8 and 9) indicate that pyroxenes were replaced by mixtures of CaFeSi O (hedenbergite) + Mn2+ CaMnSi O (johannsenite) + Fe2+ hydrous silicates, carbonates, quartz2 6 and magnetite, reflecting an increase in water2 6 activity and (3) ◦ oxygen and/or CO2 fugacities: rH25 C, 1atm = −7 kJ/mole *

5 CaFeSi2O6 (hedenbergite) + H2O + 3 CO2 (g) ↔ Ca2Fe5Si8O22(OH)2 (ferroactinolite) + 3 * Calculated for standard conditions using thermodynamic data published(4) by [59,60]. CaCO3 (calcite)+ 2 SiO2 (quartz) In contrast to the carbonate component of the host cipollino marble that was replaced by

CaFeSi2O6 (hedenbergite) + 2 H2O + CO2 (g) ↔ Ca2Fe3(SiO4)3(OH) (piemontite) + CaCO3 pyroxenes, grey mica has not been signiﬁcantly affected by metasomatic processes(5) during the prograde stage of the mineralization,(calcite) + probably3 SiO2 (quartz) due + to3 H the+ insufﬁcient water activity and a relatively high K+/H+ molar ratio:

+ + 2 KAl2(AlSi3O10)(OH)2 (kaolinite) + 3 H2O + 2 H 3 Al2Si2O5(OH)4 (mica) + 2 K (4)

According to the fluid inclusion data, the prograde stage occurred at temperatures above 405 ◦C and at pressures above 30 MPa under the influence of moderate salinity and low density Ca-Na-chloride bearing aqueous fluids. The absence of liquid CO2 indicates that XCO2 was below 0.1 [61–63]. Due to absence of any reliable independent geothermometer and/or geobarometer, only the minimum P-T conditions can be set (Figure 14). Textural relations (Figures5,8 and9) indicate that pyroxenes were replaced by mixtures of hydrous silicates, carbonates, quartz and magnetite, reflecting an increase in water activity and oxygen and/or CO2 fugacities:

5 CaFeSi O (hedenbergite) + H O +3 CO (g) Ca Fe Si O (OH) (ferroactinolite) + 2 6 2 2 2 5 8 22 2 (5) 3 CaCO3 (calcite)+ 2 SiO2 (quartz) Geosciences 2018, 8, x FOR PEER REVIEW 18 of 28

associated with the Sasa deposit are a product of the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene period [21,23,38–41,53], resulting in the formation of numerous magmatic–hydrothermal ore deposits along the Vardar Zone and the Serbo-Macedonian Massif (Figure 1; [25–35,54]). The paragenetic sequence (Figure 4) indicates that, during its formation, the Sasa Pb-Zn-Ag deposit underwent three main stages similar to other known skarn deposits worldwide: (1) a stage of isochemical metamorphism; (2) an anhydrous prograde stage and (3) a retrograde/hydrothermal stage [1,55,56]. As the deposit is hosted by a highly metamorphosed terrain, it is difficult to distinguish the regional metamorphic signature from the metamorphism associated with the emplacement of Tertiary magmatic bodies. However, the isotopic composition of preserved lenses of the host marble overlap with values typical for marine carbonates, suggesting that the metamorphism has not disturbed its primary isotopic composition. The prograde mineral assemblage is marked by the presence of anhydrous Ca-Fe-Mg-Mn silicates, predominantly pyroxenes from the hedenbergite–johannsenite series. The predominance of anhydrous minerals reflects a low water activity, whereas the prevalence of pyroxene (hedenbergite) over garnets (andradite) suggests a high ferrous/ferric ratio and a relatively reductive environment [57]. Absence of Fe-sulfides indicates a low sulfur fugacity during the prograde stage. Although our SEM/EDS analyses (Table 1) revealed that pyroxenes are characterized by a relatively high and uniform FeO/MnO ratio, previously published data [12,58] suggest greater variations in the pyroxene composition among different ore bodies and local predomination of Mn-rich pyroxenes. The variations may be controlled by periodical variations in the chemistry of infiltrating magmatic fluids:

CaCO3 (calcite) + 2 H4SiO4 + Fe2+ ↔ CaFeSi2O6 (hedenbergite) + 3 H2O + CO2 + 2 H+ (1)

CaCO3 (calcite) + 2 H4SiO4 + Mn2+ ↔ CaMnSi2O6 (johansennite) + 3 H2O + CO2 + 2 H+ (1) and/or in local temperature oscillations:

CaFeSi2O6 (hedenbergite) + Mn2+ ↔ CaMnSi2O6 (johannsenite) + Fe2+ (2) rH25 °C, 1atm = −7 kJ/mole * * Calculated for standard conditions using thermodynamic data published by [59,60].

In contrast to the carbonate component of the host cipollino marble that was replaced by pyroxenes, grey mica has not been significantly affected by metasomatic processes during the Geosciences 2018, 8, 444 19 of 28 prograde stage of the mineralization, probably due to the insufficient water activity and a relatively high K+/H+ molar ratio:

2 KAl2(AlSi3O10)(OH)2 (kaolinite) + 3 H2O + 2 H+ ↔ 3 Al2Si2O5(OH)4 (mica) + 2 K+ (3) CaFeSi2O6 (hedenbergite) + 2 H2O + CO2 (g) Ca2Fe3(SiO4)3(OH) (piemontite) + + (6) According to the fluid inclusionCaCO3 data,(calcite) the prograde + 3 SiO stage2 (quartz) occurred + at 3temperatures H above 405 °C and at pressures above 30 MPa under the influence of moderate salinity and low density Ca-Na- chloride bearing aqueous fluids. The absence1 of liquid CO2 indicates that XCO2 was below 0.1 3 CaFeSi2O6 (hedenbergite) + 2 O2 (g) + 3 CO2 (g) 3 CaCO3 (calcite) + [61–63]. Due to absence of any reliable independent geothermometer and/or geobarometer, only the (7) FeO·Fe2O3 (magnetite) + 6 SiO2 (quartz) minimum P-T conditions can be set (Figure 14). Textural relations (Figures 5, 8 and 9) indicate that pyroxenes were replaced by mixtures of CaMgSi O (diopside) + 2 H O + 2 CO (g) Mg Si O (OH) (chrysotile) + hydrous silicates,2 6 carbonates, quartz and2 magnetite,2 reflecting an3 increase2 5 in water4 activity and (8) oxygen and/or CO2 fugacities: 3 CaCO3 (calcite)+ 4 SiO2 (quartz)

5 CaFeSi2O6 (hedenbergite) + H2O + 3 CO2 (g) ↔ Ca2Fe5Si8O22(OH)2 (ferroactinolite) + 3 Although minerals formed during the prograde phase do not incorporate Al,(4 the) retrograde mineral paragenesis contains aluminosilicatesCaCO3 (calcite)+ 2 SiO (mostly2 (quartz) chlorites and epidote group minerals). The presence of carbonates points to near-neutral pH conditions and a limited capability for hydrothermal CaFeSi2O6 (hedenbergite) + 2 H2O + CO2 (g) ↔ Ca2Fe3(SiO4)3(OH) (piemontite) + CaCO3 (5) transport of aluminum [64]. However,(calcite) layers + 3 SiO2 of (quartz) grey mica+ 3 H+ within the host cipollino marble might have served as a local source of Al:

+ 15 CaMgSi2O6 (dioside) + 2 KAl2(AlSi3O10)(OH)2 (mica) + 5 H2O + 15 CO2 + 10 H (9) 3 Mg5Al(AlSi3)O10(OH)8 (clinochlore) +15 CaCO3 (calcite) + 25 SiO2 (quartz)

6 CaFeSi O (hedenbergite) + 2 KAl (AlSi O )(OH) (mica) + H O 2 6 2 3 10 2 2 (10) 3 Ca2FeAl2Si3O12(OH) (epidote) + FeO·Fe2O3 (magnetite) + 9 SiO2 (quartz) The stable isotope composition of the carbonates revealed a significant contribution of magmatic CO2 during the retrograde stage of the Sasa deposit (Figure 13). The fluid inclusion studies suggest that infiltrating fluids were Mg-Na-chloride or Fe2+-chloride solutions. Their salinities are slightly greater compared to the prograde fluids and the homogenization temperatures point to the gradual cooling of the system. The coexistence of rhodonite, rhodochrosite and quartz implies that temperature and pressure did not exceed 375 ◦C and 200 MPa (Figure 14). Cooling of the system below 400 ◦C resulted with the ductile-to-brittle transition [65–67], promoted reactivation of old (pre-Tertiary) faults and shifted the system from the lithostatic to hydrostatic regime. Such conditions allowed progressive infiltration of ground water and therefore increased the water activity and oxygen fugacity. At the same time, due to the lithostatic to hydrostatic transition, the pressure dropped by approximately 2.7 times, triggering a more efficient degassing of the emplaced magmatic body and increasing fugacity of numerous volatiles including H2O, CO2,H2S and/or SO2. The progressive contribution of magmatic CO2 has been recognized from the retrograde mineral paragenesis (Equations (5)–(9); Figure4), as well as from the isotopic composition of associated carbonates (Table4; Figure 13). As Cl preferentially partitions into the fluid phase [68,69], fluxes of magmatic fluids will increase the total salinity of the circulating fluids. This increase in the salinity during the retrograde stage has been recorded by fluid inclusions entrapped by retrograde quartz, i.e., quartz that crystallized as the retrograde alteration product after prograde pyroxenes (Figure 15). The greater salinity promoted the metal–chloride complexing which, together with the higher water activity in the system, enhanced the hydrothermal transport of base metals, including Pb and Zn [64], and moved the Sasa deposit to the hydrothermal ore-forming stage (Figure4). The retrograde stage is marked by an increase in the sulfur fugacity that resulted in a replacement of Fe-silicates, mostly hedenbergite, by pyrrhotite during the early retrograde stage and by pyrite during the later retrograde stage and the hydrothermal stage (Figure4). The previously published sulfur isotope data show that sulfur is predominantly magmatic in origin [70]. The textural features also point to a slight time lag between intensive degassing of magmatic CO2 and S-bearing volatiles, probably controlled by the difference in solubility of CO2 and S in silicate melts [71]. Geosciences 2018, 8, x FOR PEER REVIEW 19 of 28

3 CaFeSi2O6 (hedenbergite) + ½ O2 (g) + 3 CO2 (g) ↔ 3 CaCO3 (calcite)+ FeO‧Fe2O3 (6) (magnetite) + 6 SiO2 (quartz)

CaMgSi2O6 (diopside) + 2 H2O + 2 CO2 (g) ↔ Mg3Si2O5(OH)4 (chrysotile) + 3 CaCO3 (7) (calcite)+ 4 SiO2 (quartz) Although minerals formed during the prograde phase do not incorporate Al, the retrograde mineral paragenesis contains aluminosilicates (mostly chlorites and epidote group minerals). The presence of carbonates points to near-neutral pH conditions and a limited capability for hydrothermal transport of aluminum [64]. However, layers of grey mica within the host cipollino marble might have served as a local source of Al:

15 CaMgSi2O6 (dioside) + 2 KAl2(AlSi3O10)(OH)2 (mica) + 5 H2O + 15 CO2 + 10 H+ ↔ 3 (8) Mg5Al(AlSi3)O10(OH)8 (clinochlore) +15 CaCO3 (calcite) + 25 SiO2 (quartz)

Geosciences6 CaFeSi20182O6, 8(hedenbergite), 444 + 2 KAl2(AlSi3O10)(OH)2 (mica) + H2O ↔ 3 Ca2FeAl2Si3O12(OH) 20 of(9 28) (epidote) + FeO‧Fe2O3 (magnetite) + 9 SiO2 (quartz)

Geosciences 2018, 8, x FOR PEER REVIEW 20 of 28 cooling of the system. The coexistence of rhodonite, rhodochrosite and quartz implies that temperature and pressure did not exceed 375 °C and 200 MPa (Figure 14). Cooling of the system below 400 °C resulted with the ductile-to-brittle transition [65–67], promoted reactivation of old (pre-Tertiary) faults and shifted the system from the lithostatic to hydrostatic regime. Such conditions allowed progressive infiltration of ground water and therefore increased the water activity and oxygen fugacity. At the same time, due to the lithostatic to hydrostatic transition, the pressure dropped by approximately 2.7 times, triggering a more efficient degassing of the emplaced magmatic body and increasing fugacity of numerous volatiles including H2O, CO2, H2S and/or SO2. The progressive contribution of magmatic CO2 has been recognized from the retrograde mineral paragenesis (Equations (5)–(9); Figure 4), as well as from the isotopic composition of associated carbonates (Table 4; Figure 13). As Cl preferentially partitions into the fluid phase [68,69], fluxes of magmaticFigure fluids 14.14. Pressure–temperaturePressure will increase–temperature the total (P–T)(P–T) salinity diagram of showing the circulating the ranges fluids. of isochores This obtainedincrease fromin the fluidfluid salinity duringinclusionsinclusions the retrograde thatthat hosthost progradestage has fluidsfluids been (primary recorded fluidfluid by inclusions fluid inclusions in pyroxene), entrapped retrograde by fluidsfluidsretrograde (primary quartz , i.e., quartzfluidfluid inclusionsinclus that ionscrystallized inin retrograderetrograde as the quartz),quartz), retrograde hydrothermal/ore-bearinghydrothermal/ore alteration product-bearing after fluids fluids prograde (primary pyroxenes fluidfluid inclusionsinclusions (Figure inin 15). The greatersynoresynore quartz),quartz), salinity hydrothermal/post-orehydrothermal/post promoted the metal-ore fluids– fluidschloride (primary complexing fluid fluid inclusions which, in together post-orepost-ore with calcite) the and higher a very water activitylatelate in hydrothermal hydrothermal the system, (secondary)enhanced (secondary) the fluid fluid hydrothermal inclusion inclusion in in post-oretr postansport-ore calcite. calcit of basee. The metals, trapping including conditions conditions Pb an ford the theZn [64], and moveretrograderetrograded the stagestage Sasa areare deposit limitedlimited to byby the thethe hydrothermal P–TP–T stabilitystability ofof ore rhodonite,rhodonite,-forming rhodochrositerhodochrosite stage (Figureand and 4). quartz. quartz.

The stable isotope composition of the carbonates revealed a significant contribution of magmatic CO2 during the retrograde stage of the Sasa deposit (Figure 13). The fluid inclusion studies suggest that infiltrating fluids were Mg-Na-chloride or Fe2+-chloride solutions. Their salinities are slightly greater compared to the prograde fluids and the homogenization temperatures point to the gradual

FigureFigure 15.15. CorrelationCorrelation of of homogenization homogenization temperature temperature and and salinity salinity for for ﬂuid fluid inclusions inclusions in skarn,in skarn, syn-ore syn- andore and post-ore post- minerals.ore minerals.

The retrograde stage is marked by an increase in the sulfur fugacity that resulted in a replacement of Fe-silicates, mostly hedenbergite, by pyrrhotite during the early retrograde stage and by pyrite during the later retrograde stage and the hydrothermal stage (Figure 4). The previously published sulfur isotope data show that sulfur is predominantly magmatic in origin [70]. The textural features also point to a slight time lag between intensive degassing of magmatic CO2 and S-bearing volatiles, probably controlled by the difference in solubility of CO2 and S in silicate melts [71]. The isotopic composition of hydrothermal carbonates reflects diminishing influence of magmatic CO2 and more significant contribution of the host cipollino marble (Figure 13). The fluid inclusions studies revealed that the syn-ore mineralization was deposited from Mg-Na-chloride or Fe2+-chloride hydrothermal fluids. The wide range of recorded homogenization temperatures, together with the variable salinities (Figure 15), suggest cooling under the influence of cold ground waters as a plausible mechanism for the ore deposition:

PbCl2 (aq) + H2S (aq) ↔ PbS + 2 H+ + 2 Cl− (10)

Geosciences 2018, 8, x FOR PEER REVIEW 18 of 28

CaCO3 (calcite) + 2 H4SiO4 + Fe2+ ↔ CaFeSi2O6 (hedenbergite) + 3 H2O + CO2 + 2 H+ (1)

GeosciencesCaCO20183 ,(calcite)8, 444 + 2 H4SiO4 + Mn2+ ↔ CaMnSi2O6 (johansennite) + 3 H2O + CO2 + 2 H+ (1) 21 of 28 and/or in local temperature oscillations:

CaFeSi2O6 (hedenbergite) + Mn2+ ↔ CaMnSi2O6 (johannsenite) + Fe2+ The isotopic composition of hydrothermal carbonates reflects diminishing influence(2) of magmatic CO2 and more significant contributionrH25 °C, 1atmof = − the7 kJ/mole host *cipollino marble (Figure 13). The fluid inclusions studies revealed* Calculated that the for standard syn-ore conditions mineralization using thermodynamic was deposited data published from by Mg-Na-chloride [59,60]. or Fe2+-chloride hydrothermalIn contrast fluids. to the The carbonate wide component range of recorded of the host homogenization cipollino marble that temperatures, was replaced by together with the variablepyroxenes, salinities grey mica(Figure has 15 not), suggestbeen significantly cooling affected under the by metasomatic influence of processes cold ground during waters the as a plausible mechanismprograde stage for theof the ore mineralization, deposition: probably due to the insufficient water activity and a relatively highGeosciences K+/H+ molar 2018, 8ratio:, x FOR PEER REVIEW 21 of 28

2 KAl2(AlSi3O10)(OH)2 (kaolinite) + 3 H2O + 2 H+ ↔ 3 Al2Si2O5(OH)+ 4 (mica)− + 2 K+ (3) PbClZnCl2 (aq)2 (aq) + H+ 2HS2S (aq) (aq) ↔PbS ZnS + + 22 HH+ ++ 2 2Cl Cl− (11) (11) According to the fluid inclusion data, the prograde stage occurred at temperatures above 405 °C and at pressuresHowever, above neutralization 30 MPa under of the the mineralizing influence of moderate fluids in salinityreactions +and with low host−density carbonates Ca-Na- contributed ZnCl2 (aq) + H2S (aq) ZnS + 2 H + 2 Cl (12) chlorideto the orebearing dep ositionaqueous ( Figurefluids. 16):The absence of liquid CO2 indicates that XCO2 was below 0.1 [61However,–63]. Due to neutralization absence of any reliable of the independent mineralizing geothermometer ﬂuids in reactions and/or geobarometer, with host only carbonates the contributed PbCl2 (aq) + H2S (aq) + 2 CaCO3 ↔ PbS + 2 HCO3− + 2 Ca2+ + 2 Cl− (12) to theminimum ore deposition P-T conditions (Figure can be 16 set): (Figure 14). Textural relations (Figures 5, 8 and 9) indicate that pyroxenes were replaced by mixtures of 2 2 3 ↔ 3− 2+ − (13) hydrous silicates, carbonates,ZnCl (aq) quartz + H andS (aq) magnetite + 2 CaCO, reflecting ZnS an + increase2 HCO in + water2 Ca activity + 2 Cl and PbCl (aq) + H S (aq) + 2 CaCO PbS + 2 HCO − + 2 Ca2+ + 2 Cl− (13) oxygen Theand/or post CO-ore2 fugacities:2 stage is marked2 by the abundant3 deposition of calcite3 , revealing near-neutral pH

conditions.5 CaFeSi2O6 (hedenbergite)The isotopic composition+ H2O + 3 CO2 of(g) post ↔ Ca-ore2Fe carbonates5Si8O22(OH)2 overlaps(ferroactinolite) with the+ 3 values obtained from ZnCl (aq) + H S (aq) + 2 CaCO ZnS + 2 HCO − + 2 Ca2+ + 2( Cl4) − (14) syn-ore carbonates2 (FigureCaCO 123).3 (calcite)The fluid+ 2 inclusionSiO2 (quartz)3 data suggest deposition3 temperatures below 300 °C from relatively diluted Ca-Na-Cl-bearing fluids. CaFeSi2O6 (hedenbergite) + 2 H2O + CO2 (g) ↔ Ca2Fe3(SiO4)3(OH) (piemontite) + CaCO3 (5) (calcite) + 3 SiO2 (quartz) + 3 H+

FigureFigure 16. ( a16.) Solubility (a) Solubility of Pbof Pb as as a functiona function of of temperature,temperature, pH, pH, and and the the fluid fluid salinity; salinity; (b) Solubility (b) Solubility of of Zn as a function of temperature, pH, and the fluid salinity (s). The H2S fugacity (fH2S) has been fixed Zn as a function of temperature, pH, and the fluid salinity (s). The H2S fugacity (f H2S) has been fixed −as6 10−6 and the total pressure (pfluid) as 20 MPa. as 10 and the total pressure (pfluid) as 20 MPa. 6. Comparison with Other Distal Pb-Zn-Ag Skarn Deposits The mineralization at the Sasa Pb-Zn-Ag deposit shows many distinctive features typical of base metal skarn deposits including: (1) a carbonate lithology as the main immediate host of the mineralization; (2) a close spatial relation between the mineralization and magmatic bodies of an

Geosciences 2018, 8, 444 22 of 28

The post-ore stage is marked by the abundant deposition of calcite, revealing near-neutral pH conditions. The isotopic composition of post-ore carbonates overlaps with the values obtained from syn-ore carbonates (Figure 13). The ﬂuid inclusion data suggest deposition temperatures below 300 ◦C from relatively diluted Ca-Na-Cl-bearing ﬂuids.

6. Comparison with Other Distal Pb-Zn-Ag Skarn Deposits The mineralization at the Sasa Pb-Zn-Ag deposit shows many distinctive features typical of base metal skarn deposits including: (1) a carbonate lithology as the main immediate host of the mineralization; (2) a close spatial relation between the mineralization and magmatic bodies of an intermediate composition; (3) a presence of the prograde anhydrous Ca-Fe-Mg-Mn-silicate and the retrograde hydrous Ca-Fe-Mg-Mn ± Al-silicate mineral assemblages; (4) a deposition of base metal sulﬁdes, predominately galena and sphalerite, during the hydrothermal stage; and (5) a post-ore stage characterized by the deposition of a large quantity of carbonates [62,72,73]. However, mineralogical, geochemical and isotope characteristics of the Sasa Pb-Zn-Ag deposit differ in some details from similar deposits elsewhere, including the Trepca Pb-Zn-Ag deposit which represents a product of the same magmatic event (Figure2;[31]):

(1) Although the prograde mineralization of the Sasa Pb-Zn-Ag deposit can be described as a Ca-Fe-Mg-Mn-system, the retrograde mineralization shows a significant hydrothermal input of Al, plausibly reflecting a contribution of the aluminosilicate component within the host cipollino marble. (2) Prograde skarn mineralization of the great majority of skarn deposits indicates a dominance of high temperature hypersaline magmatic fluids, whereas the later retrograde stage usually reflects mixing with lower temperature and lower salinity fluids of a meteoric origin [4,62,72, 73]. However, fluid inclusions and stable isotope data obtained from the retrograde mineral assemblage of the Sasa Pb-Zn-Ag deposit suggest that in this deposit, magmatic fluids played a significant role during the retrograde stage. (3) The transition from the prograde to the retrograde stage in different skarn deposits can be triggered by various reasons, including brecciation [31] and reactivation of old fractures [62]. In the Sasa Pb-Zn-Ag deposit, the transition was initiated by cooling of the system below 400 ◦C and the resulting ductile-to-brittle transition.

7. Conclusions The geological setting of the Sasa Pb-Zn-Ag skarn deposit and previously published geochemical studies on the associated magmatic rocks (trachytic to trachydacitic composition; calc-alkaline character; Na2O/K2O < 1; high LILE/HFSE ratios; strong enrichment in K, Pb and U; the K/Ar age of 31–24 Ma) suggest that the Sasa deposit is a product of the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene period and resulted in formation of numerous magmatic-hydrothermal ore deposits along the Vardar Zone and the Serbo-Macedonian Massif. The relatively simple, pyroxene-dominated, prograde mineralization at the Sasa Pb-Zn-Ag skarn deposit resulted from an interaction of magmatic fluids with the host cipollino marble. The absence of direct contacts between the mineralization and the magmatic rocks as well as the textural features of the skarn paragenesis reflect the infiltration-driven metasomatism. Obtained mineralogical and geochemical data suggest that the prograde stage occurred under conditions of low water activity, + + low oxygen, sulfur and CO2 fugacities and a high K /H molar ratio. Fluid inclusion data set the minimum P–T conditions at 30 MPa and approximately 405 ◦C. Mineralizing fluids were moderately saline and low density Ca-Na-chloride bearing aqueous solutions. The transition from the prograde to the retrograde stage was initiated by cooling of the system below 400 ◦C and the associated ductile-to-brittle transition that shifted the system from the lithostatic Geosciences 2018, 8, 444 23 of 28 to hydrostatic regime. The retrograde mineral assemblages reflect conditions of a high water activity, + + high oxygen and CO2 fugacities, a gradual increase in the sulfur fugacity and a low K /H molar ratio. The isotopic composition of retrograde carbonates revealed a significant contribution of magmatic CO2. Infiltration fluids carried MgCl2 and had a slightly higher salinity compared to the prograde fluids. The maximum formation conditions are set to 375 ◦C and 200 MPa. The deposition of ore minerals (predominantly Bi, In and Ag- enriched galena and Fe and Mn-bearing sphalerite) occurred during the hydrothermal phase under a diminishing influence of 2+ magmatic CO2. The mixing of ore-bearing (Mg-Na-chloride or Fe -chloride) aqueous solutions with cold and diluted ground waters is the most plausible reason for the destabilization of metal–chloride complexes. However, neutralization of relatively acidic ore-bearing fluids in the interaction with the host lithology could have significantly contributed to the deposition. The post-ore, predominantly carbonate, mineralization was deposited from diluted Ca-Na-Cl-bearing fluids of a near-neutral pH composition. The depositional temperature is estimated to be below 300 ◦C.

Author Contributions: Conceptualization, S.S.P., Z.P. and L.P.; Data curation, S.S.P., G.T., D.Š., K.R., J.E.S. and K.N.; Formal analysis, D.Š., K.R., J.E.S. and K.N.; Funding acquisition, S.S.P. and L.P.; Methodology, S.S.P., J.E.S. and K.N.; Project administration, S.S.P.; Resources, Z.P., G.T. and T.S.; Supervision, S.S.P. and L.P.; Validation, T.S., K.N. and L.P.; Visualization, S.S.P., Z.P., D.Š. and K.R.; Writing–original draft, S.S.P.; Writing–review and editing, Z.P., G.T., T.S., D.Š., K.R., J.E.S., K.N. and L.P. Funding: The early stages of this study were conducted as a part of the project “Geochemical characteristics of hydrothermal ore deposits in the Republic of Macedonia” funded by Croatian Ministry of Science, Education and Sport and Macedonian Ministry of Science. The final part of the study was funded by the project A31566 at UiT The Arctic University of Norway. Acknowledgments: We would like to thank the staff at the Sasa mine, Makedonska Kamenica, for all their support during the multiple sampling campaigns. Matteus Lindgren is greatly appreciated for help in obtaining the stable isotope data. A special thank goes to Vasilios Melfos and Panagiotis Voudouris for handling the manuscript as well as to three anonymous reviewers whose comments substantially improved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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